Field of the Invention
The present invention relates to a method for measuring prevailing weather and meteorological visibility.
The present invention also concerns an apparatus for measuring prevailing weather and meteorological visibility.
An important part of weather observation is the determination of prevailing weather. The codes for prevailing weather in an international weather message include the possibility of reporting observations principally related to total cloudiness, air turbulence and type/rate of precipitation as well as different disturbance phenomena (e.g., lightning, storms).
The method and apparatus disclosed herein makes it possible to determine, on the basis of measurements, most of the parameters related to precipitation. This can appreciably promote the automation of weather observation, which in turn enables shorter intervals between weather observations and denser networks of observation points as well as their location to noninhabited regions, for instance. Precipitation in this context means precipitation of condensed moisture in all possible forms.
Accurate measurement of meteorological visibility in all weather conditions forms an essential part of weather information, in particular prevailing weather at airports.
Most conventional equipment detects only the advent and cessation of precipitation, or alternatively, measure the rate of precipitation. Analysis of precipitation in the scope dealt with in the present invention has not earlier been attempted in a single apparatus. Closest to this goal has been reached by apparatuses based on the analysis of the optical measurement signal and various types of microwave weather radars.
The key problem in precipitation analysis is the differentiation of liquid forms of water from its solid condensation forms such as hailstones and snowflakes from each other at all rates of precipitation and all weather conditions (wind, radiance, temperature). This is not possible with prior art apparatuses and methods.
Optical methods known in the art are of two types: A method based on scatter measurement of light transmitted by the apparatus and scintillation measurement of transmitted light. Furthermore, several different apparatuses based on light transmission measurement are known that are predominantly used for measurement of droplet diameter distributions.
An apparatus based on scatter measurement (U.S. Pat. No. 4,613,938) aims to detect particulate matter and measure the size and speed of particles in a sample volume which due to optical design constraints must be larger than 0.2 liters. Based on the analysis of particle sizes and speeds, a sorting matrix is formed in which large and slow particles are designated as snow, large and fast particles as hail, etc.
Only a statistical measurement of the particle size distribution is possible by means of the above-described apparatus, because the sample volume is relatively large and undefined, and the speed of the wind driving the particles can be of the same order of magnitude (approx. 5 m/s) as the falling particles gain by gravity fall. The impossibility of speed measurement was, in fact, later recognized by the patentee, after which he has added a second optical receiver to the apparatus. Yet, even a second optical receiver does not remove uncertainty from the particle speed measurement. Furthermore, the rain matrix entails overlapping of different rain types at low precipitation rates. Evidently, the sample volume required for the particle size distribution is excessively large, because even at relatively low rates of precipitation the sample volume contains several particles simultaneously.
An apparatus based on scintillation measurement determines the energy distribution at different frequencies for the scintillation of incident light from spherical particles (U.S. Pat. No. 4,760,272). According to measurements, the energy of light scintillation from water particles concentrates at higher frequencies that those encountered with snow. The intensity of scintillation allows the detection of precipitation advent and cessation as well as its rate. The speed of the particles affects the signal amplitude and quality, thereby restricting the decision making capability between different types of precipitation. Turbulence caused in windy conditions by the mechanical construction of the apparatus disturbs the measurement, and this drawback cannot be eliminated entirely. Furthermore, the shimmer effect caused by the heating of air is detected as a scintillation signal.
An apparatus based on a microwave radar aims to measure the rate of fall of particles with the help of the Doppler effect. Sorting of rain type is based on the differences in the rates of fall for the different types of precipitation. The large volume of sample space and wind can disturb the measurement. In wind, snowfall is detected as rainfall. Dripping rainfall easily is interpreted as snowfall, because the small droplets fall extremely slowly. Low rates of rainfall are not detected at all as precipitation.
Thus, conventional apparatuses permit a relatively reliable detection of the advent and cessation of precipitation alone.
Visibility measurement meters based on the measurement of light scattering operate relatively well in fog approaching drizzle, whereby their operation conforms to the scattering theory. In rainfall the particle size vastly exceeds the wavelength of incident light, thereby invalidating the preconditions of scattering theory. Furthermore, the optical properties and size distributions of snow and water differ from each other. Without corrections by type and rate of precipitation, the visibility reading of a scattering-type visibility meter is not reliable in all weather conditions.